This Small Grant will be used to gather preliminary data on the efferent innervation of the peripheral vestibular apparatus. Recent work has suggested that this population may be heterogeneous in its neurotransmitter neurochemistry and in its peripheral targets. This suggestion, if confirmed, would change our view that the efferents are a nonspecific system. The goal of this study is to define potential subpopulations of efferent neurons in the chinchilla based on their brain stem locations, their projections to one, the other or both ears, their neurochemical identities and their peripheral terminations on hair cells, calyces and other afferent processes. The specific aims are: 1) to define separate populations of efferent neurons in the brain stem based on current knowledge of possible transmitters and efferent anatomy and 2) to correlate this knowledge with an analysis of the regional synaptic relations of chemically-defined efferent boutons and fibers in the periphery. Efferent neurons in the brain stem will be identified by means of fluorescent retrograde tracer injections into the peripheral vestibular apparatus and will be simultaneously characterized immunohistochemically by the use of antibodies to several putative neurotransmitters (acetylcholine, gamma-aminobutyric acid, calcitonin gene-related peptide, met-enkephalin and others). The analysis will be extended to the periphery where electron microscopic immunohistochemical methods will be used to characterize neurochemically distinctive efferent boutons in terms of their terminations in the sensory epithelium. The significance of this project is that a precise knowledge of the different elements comprising the population of efferents will further our understanding of brain stem control of peripheral sensory processing.